The agency eavesdropped on civil rights leaders Martin Luther King and Whitney Young as well as boxing champion Muhammad Ali, New York Times journalist Tom Wicker and Washington Post columnist Art Buchwald.

Protest fears

The NSA also monitored the overseas phone calls of two prominent US senators - Democrat Frank Church and Republican Howard Baker.

Many of those targeted were considered to be critics of US involvement in the Vietnam War.

In 1967 the strength of the anti-war campaign led President Lyndon Johnson to ask US intelligence agencies to find out if some protests were being stoked by foreign governments.

The NSA worked with other spy agencies to draw up the "watch lists" of anti-war critics, tapping their phone calls.

The programme continued after Richard Nixon entered the White House in 1969. US Attorney General Elliot Richardson shut down the NSA programme in 1973, just as the Nixon administration was engulfed in the Watergate scandal.

The latest revelations come as the NSA is embroiled in fresh controversy over its surveillance programmes.

Researchers Matthew Aid and William Burr, who published the documents on Wednesday, said the spying abuses during the Vietnam War era far surpassed any excesses of the current programme.

"As shocking as the recent revelations about the NSA's domestic eavesdropping have been, there has been no evidence so far of today's signal intelligence corps taking a step like this, to monitor the White House's political enemies," they wrote.